Where are you getting GPS receivers that work in space?

Probably one of the numerous companies making qualified receivers? E.g. [0]. There's even a published table comparing basically all of them [1].

[0] https://www.aac-clyde.space/what-we-do/space-products-compon...

[1] https://www.mdpi.com/1424-8220/23/17/7648#app1-sensors-23-07...